Mobile devices such as mobile phones, PDAs (Personal Digital Assistants), and the like are nowadays progressively reduced in size and sophisticated, and a mounted structure such as a BGA (Ball Grid Array) or CSP (Chip Scale Package) is widely used to respond to such a trend. Because the mobile devices are susceptible to mechanical load such as drop impact, it is important for the mounted structure such as a BGA or CSP not having any impact relieving structure such as leads of a QFP (Quad Flat Package) to ensure the reliability of soldered joints in impact resistance.
In soldering, for example, a BGA semiconductor package and an electronic circuit board, a sealing and reinforcing technique such as an underfill for enhancing the reliability of soldered joints in impact resistance has been hitherto used in which a reinforcing resin material is filled, after soldering, in a space between the BGA semiconductor package and the electronic circuit board for fixation thereof to thereby relieve stress that may be caused by heat or mechanical impact. A thermosetting type epoxy resin has been mainly used as a conventional underfill sealing agent (see Patent Document 1, 2 or 3).
In recent years, growing concern about global environmental problems leads to practical use of solders containing no lead, i.e., Pb-free solders. Of the Pb-free solders, an Sn—Ag—Cu-based solder is nowadays widely used having relatively good wettability or high connection reliability. In addition to the Sn—Ag—Cu-based solder, a low-melting-point Pb-free solder such as, for example, an Sn—Zn-based solder, an Sn—Ag—In-based solder or an Sn—Bi-based solder begins to be used that is free from a demerit of a high melting point, as is the case with the Sn—Ag—Cu-based solder. However, the reliability in BGA connection with the use of the Sn—Zn-based solder, Sn—Ag—In-based solder or Sn—Bi-based solder is still unclear.    Patent Document 1: JP 10-101906 A    Patent Document 2: JP 10-158366 A    Patent Document 3: JP 10-204259 A